It has been conventional for many years to delignify and bleach wood pulp by the use of various chlorination procedures. These are sometimes referred to in the paper industry as the C.sub.D E stages in the 5-stage C.sub.D EDED or 6-stage C.sub.D EHDED sequences. The use of chlorine gas is not inexpensive and the removal of unused chlorine gas and the chlorine-containing by-products from the effluent streams requires expensive chemical recovery systems so as to abate stream and environmental pollution problems.
Over the years suggestions have been advanced to replace the conventional chlorine delignification and bleaching treatments by replacing the use of chlorine with oxygen. A number of processes for bleaching and delignifying pulp with oxygen have been proposed, such as Richter U.S. Pat. Nos. 1,860,432, Grangaard et al. 2,926,114 and 3,024,158, Gaschke et al. 3,274,049, Meylan et al. 3,384,533, Watanabe 3,251,730, Rerolle et al. 3,423,282, Farley 3,661,699, French Pat. Nos. 1,310,248 and 1,387,853 and articles by Nikitin et al. in Trudy Leningradshoi Lesotekb. Nickeskoi Akad. i.S.M. Korova (Transactions of the Leningrad Academy of Forestry), Vol. 75, pp. 145-155 (1956), Vol. 80, pp. 65-75, 77-90 (1958) and Bumazh. Prom., Vol. 35, No. 12, pp. 5-7 (1960). However, these processes present certain disadvantages. Many of these processes require protective agents, such as magnesium carbonate disclosed in Meylan U.S. Pat. No. 3,384,533, to prevent depolymerization of the cellulose and preservation of pulp viscosity. In addition to the imparting of scale and encrustation problems on the process equipment, the use of such chemicals has a serious disadvantage in that they present pollution abatement problems. If pollution is to be avoided, expensive recovery treatments must be employed to remove such protective agents from the effluent streams.
Roymoulik and Brown U.S. Pat. No. 3,832,276, granted Aug. 27, 1974 marked an important advance in the art because it presented a commercially feasible process for delignification and bleaching of an alkaline, dilute slurry of pulp by means of oxygen. In that process there was employed an alkaline aqueous pulp slurry of a consistency of from about 2 to 10%, having a pH of between about 9 and 14, a reaction temperature of between about 70.degree. and 120.degree.C., with the oxygen dissolved and intimately dispersed and subdivided into the slurry so that no agglomerated bubbles are formed and the oxygenated pulp slurry has substantially no bubbles exceeding about 1/16 inch in diameter. Conditions were employed so as to gradually decrease the pressure to which the slurry is subjected and continuously withdrawing treated slurry from the system. In an optional feature of said process, the slurry is pretreated with oxygen and alkali at an elevated temperature and pressure in a pretreatment vessel.
The aforesaid process of said U.S. Pat. No. 3,832,276, provided the paper industry with a new, efficient, continuous process which made excellent use of the chlorination towers with which conventional paper making plants were already equipped. In accordance with the present invention, I have provided an improvement over the process of said patent, making optimum use of the pressurized pretreatment stage of said process.
It is well established in the literature of oxygen bleaching that, all other variables held constant, an increase in reaction temperature will result in an increase in the extent of delignification. An article by Jan Gajdos in Papir a Celluloza, March, 1973, pp. 15-20, clearly demonstrates this point. When employing the conventional tower and passing the oxygenated pulp slurry upward through the tower, however, the maximum attainable temperature at the top of the tower is the boiling temperature of the alkaline pulp slurry. Since heat loss up the tower is small, the temperature at the base of the tower would also be near boiling. To use higher temperature would result in the highly undesirable consequence of flashing, and the belching of slugs of pulp slurry up the length of the tower, which cannot be tolerated.
It is, accordingly, an object of the present invention to provide a commercial low-cost continuous process for the delignification and bleaching of wood pulp, which is an improvement over the process of U.S. Pat. No. 3,832,276.
It is another object of the present invention to provide a low-cost continuous process for delignification and bleaching wood pulp which provides a practical method of achieving higher temperature in one portion of the process and, consequently, greater delignification.
It is a further object of the present invention to provide a process for delignification and bleaching of wood pulp which provides an additional benefit in the reduction of the amount of carbohydrate degradation, or viscosity loss, that accompanies delignification. This benefit is obtained as a result of the lower concentrations of NaOH that are utilized.
It is an additional object of the present invention to provide a low-cost continuous process for delignification and bleaching of wood pulp by the use of oxygen which provides an additional benefit of the invention in that, for a given retention time in the pre-retention reactor, use of the present invention allows the size of the reactor, and hence its cost, to be reduced.